U.S. Pat. No. 4,483,202 discloses an ultrasonic flow meter. The flow meter comprises a conduit through which a fluid flows and a pair of ultrasonic transducers disposed in the conduit along a line which is inclined by a predetermined angle with respect to a fluid flowing direction. In the operation, initially, an ultrasonic wave is emitted from the ultrasonic transducer disposed on the upstream side with respect to the flowing direction, toward the downstream side, and is received by the ultrasonic transducer on the downstream side. Then the propagation time from the emission to the reception of the ultrasonic wave is determined. Subsequently, an ultrasonic wave is emitted from the ultrasonic transducer disposed on the downstream side with respect to the flowing direction, toward the upstream side, and is received by the ultrasonic transducer on the upstream side, and the propagation time from the emission to the reception of the ultrasonic wave is determined. Substituting the two propagation times into a well-known equation yields the velocity of the fluid, with which the rate of flow is determined.
In such a well-known ultrasonic flow meter as described above, both velocity and rate of flow are measured by emitting ultrasonic waves at a predetermined time interval regardless of the flow rate. As a result, the conventional ultrasonic flow meter has a large power consumption, which causes a battery to run down in a short period of time.